Direct printing of liquid metal 3D microstructures

(Nanowerk Spotlight) The ability to pattern materials into arbitrary three-dimensional (3D) microstructures is important for electronics, microfluidic networks, tissue engineering scaffolds, photonic band gap structures, and chemical synthesis. However, existing commercial processes to 3D print metals usually require expensive equipment and large temperatures. In contrast, a novel, relatively simple method developed by researchers at North Carolina State University can print metal structures at room temperature. This makes the technique it compatible with many other materials including plastics. Also, the resulting structures are liquid and are therefore soft and stretchable.

"The key concept is that the liquid metal forms spontaneously a thin oxide layer on its surface," Michael Dickey, an Associate Professor of chemical and biomolecular engineering at NC State, tells Nanowerk. This oxide layer is solid and allows the metal to be printed into 3D shapes despite being a liquid. When two droplets of water come together, they form a larger droplet. However, this does not happen with the liquid metal due to the oxide 'skin'."

As the team reports in a recent issue of Advanced Materials ("3D Printing of Free Standing Liquid Metal Microstructures"), they have demonstrated that it is possible to direct write structures composed of a low-viscosity liquid with metallic conductivity at room temperature. The liquid metal is useful for soft, stretchable, or shape reconfigurable electronics.

Metals have unique electrical, optical, and thermal properties. With this novel technique, it is now possible to print metal microstructures directly to creates various parts including electronics. The resulting parts, if designed correctly, can be stretchable.

The general approach for printing liquid metal structures involves applying modest gauge pressure to a syringe needle that then extrudes the liquid metal – for this work they used the binary eutectic alloy of gallium and indium but they say that any alloy of gallium will also work – onto a substrate controlled by a motorized translation stage.

Upon exposure to air, the metal forms a thin (∼1 nanometer) passivating 'skin' composed of gallium oxide. This oxide skin on the surface of the metal stabilizes the liquid metal wire against gravity and surface tension of the liquid. Once detached from the syringe, the wires maintain their shape.

3D printing of liquid metals at room temperature.

"The formation of the wires is remarkable and unexpected" says Dickey. "The process of forming the wires begins by forming a bead of the metal on the tip of the syringe. Although the metal is under pressure the entire time, it does not flow out of the syringe due to the stabilizing influence of the oxide skin. Without increasing or decreasing the pressure in the syringe, wires form when the metal contacts the substrate and the tip of the syringe withdraws away from the substrate. Because the oxide skin spans from the nozzle of the syringe to the substrate, increasing the distance between the nozzle and substrate generates a tensile force along the axis of the wire that yields the skin and allows the wire to elongate. The pressure of the liquid metal retards any destabilizing capillary forces long enough for new skin to form and thereby mechanically stabilizes the wire."

Altogether, the researchers describe four different methods to direct write 3D, free standing, liquid metal microstructures by extruding the liquid metal through a capillary: "In addition to extruding wires, it is possible to form free standing liquid metal microstructures using at least three additional methods," Dickey explains: "1) Expelling rapidly the metal to form a stable liquid metal filament; 2) stacking droplets; and 3) injecting the metal into microchannels and subsequently removing the channels chemically."

The smallest components that the team fabricated were about 10 µm, but they note that there may be opportunities to create smaller structures through, for example, the use of smaller nozzles.

Dickey?s team is currently exploring how to further develop these techniques, as well as how to use them in various electronics applications and in conjunction with established 3-D printing technologies.

Dickey notes that the work by an undergraduate student, Collin Ladd, also the paper's first author, was indispensable to this project. ?He helped develop the concept, and literally created some of this technology out of spare parts he found himself.?